It depends who you are planning to fly with? BA will allow you more than one bike, but you'll need to pay for the second @ £35 at the airport, or £28 in advance (when booking), Easyjet allow only one piece of sporting equipment, don't know about Ryanair, but guess they will be similar to Easyjet.
You'll need to partially dismantle the bikes, and pack them in bike bags.
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